Some mammals, including primates, express dominant restriction factors that block the infection of certain retroviruses during the early phase of virus replication. TRIM51 or, in some species, a TRIM5- cyclophilin A fusion protein called TRIMCyp, mediates these early blocks by binding the retroviral capsid as it enters the host cell cytoplasm. TRIM51 is a member of the tripartite motif (TRIM) family of proteins and contains RING, B-box 2, coiled coil and B30.2/SPRY domains. The coiled coil contributes to TRIM51 dimerization; the B30.2/SPRY domain contributes to capsid recognition. The functions of the RING and B-box 2 domains in TRIM51-mediated retroviral restriction are less well understood. These domains contribute to the potency of retrovirus restriction, which is thought to involve the accelerated uncoating the viral capsid. The overall goal of this competing renewal application is to understand the molecular mechanism of TRIM51- mediated retrovirus restriction and to define the roles of individual TRIM51 domains and capsid elements in virus recognition and inhibition. The specific aims of this proposal are: 1) To investigate the role of the TRIM51 B-box 2 domain in retrovirus restriction; 2) To investigate the contribution of the E3 ubiquitin ligase activity of the TRIM51 RING domain to retrovirus restriction; 3) To test mechanistic models of TRIM51-mediated retrovirus restriction; and 4) To define how changes in the retroviral capsid influence interaction with TRIM51 and host cell cofactors such as cyclophilin A.